The approval of the anti-vascularendothelial growth factor (VEGF) antibody, bevacizumab (Avastin), as the first anti-angiogenic agent heralds a new era in the treatment of solid tumors, but raises many questions as to how to optimally use these agents. During the current grant period, we have attempted to resolve an apparent paradox in anti-angiogenic therapy - how can an anti-angiogenic agent (which destroys tumor vessels) enhance the outcome of cytotoxictherapy (which requires blood vessels for the delivery of drugs or oxygen)? We have discovered that anti-angiogenic treatment can "normalize" the abnormal structure and function of tumor vessels, making them more efficient for the delivery of therapeutic agents. We have also found that radiotherapy has an enhanced effect against orthotopically grown glioblastoma (a brain tumor) when it is given within the "normalization window" - a period during anti-angiogenic treatment when vascular function transiently improves. Furthermore, our data indicate that perivascular (mural) cell recruitment stabilizes tumor blood vessels. These provocative findings, supported by data from other laboratories and by our clinical data, raise two critical questions which we propose to address: (i) to what extent, and for how long, is the tumor vasculature normalized by other anti-angiogenic agents that are currently being tested in the clinic? And (ii) do strategies which facilitate perivascular cell recruitment also improve vascular function and enhance response to cytotoxic therapies? Using our established preclinical models, we will investigate the response to agents currently in use in clinical trials! We will begin by comparing the vascular normalization produced by low-molecular-weight tyrosine kinase inhibitors with that produced by antibodies. These agents are known to vary in their degree of VEGF and platelet derived growth factor (PDGF) signal inhibition (Aim 1). We will then attempt to extend the normalization window produced by these agents by modulating the Tie2 pathway via overexpression of Ang-1 (Aim 2). Finally, we will combine VEGF blockade with enhanced PDGF receptor-psignaling in order to improve perivascular cell coverage of tumor vessels and enhance tumor vascular function and response to radiotherapy (Aim 3). Using these new strategies, we seek to overcome physiological barriers that impede delivery of therapeutic agents to solid tumors, and to collaborate with physicians to develop improved, clinically relevant anti- angiogenic treatment strategies.